The present invention relates to an extrudable composition that can be cross-linked in air, and also to a method of cross-linking said composition.
This composition is for use in the manufacture of sheathing material for medium and low-voltage power cables or for telecommunications cables. These cables carry direct current (DC) or alternating current (AC). Medium-voltage cables are generally constituted by a conductive core surrounded by an insulating structure that is coaxial thereabout. The structure comprises at least a semiconductive first layer placed in contact with the core of the cable, itself surrounded by an electrically insulative second layer, in turn covered by a semiconductive third layer. The outer layers are sheaths which serve to protect the cable. Low-voltage cables (voltage lower than 20 kilovolts (kV)) have a conductive core surrounded by an insulating structure which is coaxial thereabout and covered in a sheath. This composition is also usable as an insulating material.
In the event of a short circuit occurring in a cable, temperatures locally rising to 200xc2x0 C. can be observed. At these temperatures and under stress, the material is subject to deformation or creep. This elongation when hot is evaluated using the so-called xe2x80x9chot set testxe2x80x9d (HST) under conditions which are defined in standard NF EN 60811-2-1. Compositions are being sought which present good results in the xe2x80x9chot set testxe2x80x9d.
Document WO-88/05449 proposes in its examples 11 and 12 an extrudable composition which comprises a mixture of a copolymer of an ethylenically-unsaturated compound and an unsaturated ester containing an acrylate such as ethylene butylacrylate (EBA) (example 11) or an ethylene methylacrylate (EMA) (example 12) with a hydrolizable silane compound having no ethylenic unsaturation, such as a 3-amino-propyltrimethoxysilane (MEAM) and also with a thermoplastic polymer having no functional groups such as a polyethylene homopolymer, e.g. low density polyethylene (LDPE).
The resulting material presents good adhesion on metals or polar substances and it is used for this purpose in cables. However that material is not cross-linkable.
Document EP-A-0 802 224 describes an extrudable composition that is cross-linkable, and that is constituted by a mixture comprising a thermoplastic polymer material, a hydrolizable silane compound, a non-cross-linked elastomer, and a cross-linking agent. The thermoplastic polymer material and the hydrolizable silane compound respectively carry mutually reactive functional groups so that they react together selectively. The elastomer is cross-linked dynamically and selectively by the cross-linking agent in the mixture. The thermoplastic polymer material carrying reactive functional groups is a commercially-available substance, preferably polyethylene grafted by maleic anhydride. The silane compound is preferably aminoalkoxysilane. The amine groups of the silane compound react selectively with the maleic anhydride groups of the polyethylene, and the alkoxy groups of the silane compound react with the surface hydroxyl groups of the filler. The elastomer is an ethylene vinyl acetate (EVA) copolymer which is cross-linked dynamically and selectively by the cross-linking agent.
The special feature of that composition is the choice of basic components in the mixture which have specific and different reaction kinetics, thereby enabling the composition to be made in a single stage. As a result, the choice of basic components is very restricted and the cost thereof is high.
An object of the present invention is to propose an extrudable composition that is cross-linkable in air to give a cross-linked material which is recyclable and which is simpler to manufacture, and which is of lower cost than known compositions.
In a first aspect, the present invention thus provides a composition that is extrudable and cross-linkable in air, comprising a mixture:
of a copolymer of an ethylenically-unsaturated compound and an unsaturated ester;
of a hydrolizable silane compound having no ethylenic unsaturation and containing functional groups; and
of a thermoplastic polymer;
wherein said functional groups of said ester are selectively reactive with functional groups of said silane compound, and wherein said thermoplastic polymer has no functional groups.
The invention is based on the discovery that under certain conditions it would appear that a selective reaction occurs between the functional groups of the ester of the copolymer and the reactive functional groups carried by the silane.
The composition of the invention comprises a thermo-plastic polymer that has no functional groups liable to interfere with the reaction between the silane and the copolymer involved in cross-linking the composition. This thermoplastic copolymer confers good mechanical properties on the composition of the invention since it contributes to reinforcing its structure.
The present invention makes it possible to use a vast range of silane compounds. In addition, it does not require recourse to polymers having special functional groups, and consequently it makes it possible to use a vast range of polymers, and in particular polymers that are the most widespread and the least expensive.
The present invention makes it possible to obtain a composition having good resistance to creep at high temperature.
In addition, a composition of the invention makes it possible to obtain finished products which can be recycled. It also makes it easy to obtain finished products that are very flexible, for example cables based essentially on (poly)vinyl acetate.
In a second aspect, the present invention provides a method of preparing a composition which enables the composition of the invention to be cross-linked, and including in particular a xe2x80x9cdynamic cross-linkingxe2x80x9d step.
This step serves in particular to lengthen the chains of the polymer and of the copolymer and thus reduce the number of chains. In this way, this step makes it possible to improve compatibility between the polymer and the copolymer by promoting cross-linking of the composition.
In another aspect, the present invention provides a cable whose insulation and/or sheath contain the composition obtained by the method.
Other characteristics and advantages of the invention are described below in detail in the following description which is given by way of non-limiting illustration.
Composition of the invention
A composition of the invention comprises in particular a copolymer of an ethylenically-unsaturated compound and an unsaturated compound and unsaturated ester.
The selective reaction between the silane and the ester on which the invention is based can be illustrated by the following scheme:
SiX4+4RCOORxe2x80x2xe2x86x92Si(ORxe2x80x2)4+4RCOX 
in which:
X represents a functional group carried by the silane SiX4; and
RCOORxe2x80x2 represents a fragment of the copolymer of an ethylenically-unsaturated compound and an unsaturated ester.
In the invention, the unsaturated ester of the copolymer generally satisfies the formula R1COOR2 in which:
R1 is an alkyl group preferably having 1 to 4 carbon atoms; and
R2 is an alkylene group preferably having 2 to 4 carbon atoms.
Advantageously, the functional groups of the ester of the invention can comprise acetates. Acetates promote the above-mentioned reaction, unlike other groups such as acrylates.
A composition of the invention preferably also comprises a cross-linking agent which can be a peroxide, a phenolic resin, a sulfur-containing derivative, or a mixture of these compounds, in particular. The composition of the invention generally contains 0.5 to 7 parts by weight and preferably 0.5 to 3 parts by weight of said cross-linking agent.
In the invention, the ethylenically-unsaturated compound is preferably ethylene.
As examples of copolymers entering into the composition of the invention, mention can be made of ethylene vinyl acetate copolymers (EVA), ethylene vinyl propionate copolymers, ethylene allyl acetate copolymers, and ethylene allyl propionate copolymers.
It is preferable to use an ethylene vinyl acetate copolymer (EVA) containing up to 80% by weight vinyl acetate, and more specifically an EVA containing 10% to 70% by weight vinyl acetate.
Concerning proportions, the composition of the invention generally comprises 25 to 95 parts and preferably 30 to 85 parts by weight of copolymer.
In an embodiment, the hydrolizable silane having no ethylenic unsaturation of the invention can be a trialkoxysilane or a tetralkoxysilane, such as tetraethoxysilane.
It is preferable to use a silane containing an amino function, such as aminotrialkoxysilane. As an example of such a silane, mention can be made of aminopropyltriethoxysilane and of aminopropyltrimethoxysilane.
The composition of the invention generally comprises 0.5 to 5 parts and preferably 0.7 to 4 parts by weight of the silane compound.
The term xe2x80x9csilane compound having groups that are selectively reactive with the ester functional groups of the copolymerxe2x80x9d as used in the present invention means a compound which, under reaction conditions, is capable of reacting essentially with the ester functional groups of the copolymer in the scheme specified above. Thus, it is understood that the composition can include other reactive compounds, providing they do not interfere significantly with the selective reaction between the reactive groups of the silane and of the copolymer.
The thermoplastic polymer having no functional groups can be a homopolymer of an olefin having 2 to 6 carbon atoms and a polymer of 2 olefins each having 2 to 6 carbon atoms, the olefins being constituted, for example, by ethylene, propylene, butene, pentene, hexene, isobutylene, methyl-butene, methyl-pentene, dimethyl-butene, or ethyl-butene. In general, polyethylene is used, and preferably high density polyethylene (HDPE).
The proportion of polymer having no functional group in the composition generally lies in the range 0.5 to 75 parts by weight.
In a preferred embodiment, the composition of the invention can also include a filler.
This filler is preferably a filler that is not surface-reactive, such as chalk, carbon black, non-reactive magnesia, or natural or synthetic clay.
The proportion of this non-surface-reactive filler in the composition is not crucial. It generally lies in the range 0 to 230 parts by weight and usually in the range 0 to 180 parts by weight.
The composition of the invention does not require the use of fillers having reactive surfaces, and consequently it makes it possible to employ a vast range of fillers, and in particular those which are the most widely available and the least expensive.
In a variant, the composition of the invention can also contain a filler that is surface-reactive, of the type used in application EP-A-0 802 223 in the name of the Applicant. This filler can be reactive magnesia, alumina, kaolin, or mica. Since the filler is surface-reactive, it can interfere with the reaction between the silane and the copolymer.
In general, the content of said surface-reactive filler is less than 250 parts by weight, and preferably less than 160 parts by weight.
The composition is preferably free of any surface-reactive filler.
Advantageously, the composition of the invention can also include a catalyst for the reaction between the silane functional group(s) and the ester functional groups of the copolymer.
Such a catalyst can be based on an amine, a tin salt, or a molecule containing at least one atom of tin.
The proportion of catalyst in the composition of the invention generally lies in the range 0 to 500 parts per million (ppm) and more usually in the range 0 to 100 ppm.
Naturally, the composition of the invention can include various additives of the kind commonly used in fabricating cross-linked copolymers. As examples of such additives, mention can be made antioxidants, flame retarders, anti-UV agents, plasticizers, and coloring agents.
The proportions of these various additives in the composition of the invention generally lie in the range 0 to 5 parts by weight and usually in the range 0.1 to 2 parts by weight.
Once the composition of the invention has been cross-linked, it presents no significant elongation after 15 minutes (min) at 200xc2x0 C. under a stress of 0.2 mega Pascals (MPa) which are the conditions defined by standard NF EN 60811-2-1, after being stored in ambient air without taking special precautions for a period of one week.
Its hardness on the Shore A scale is preferably less than 95.
It preferably presents breaking elongation greater than 130% and more preferably of not less than 150%.
The ultimate tensile strength of the cross-linked composition of the invention is preferably greater than 7 MPa.
Method of the Invention
A cross-linked composition of the invention can be prepared in a single dynamic cross-linking step in the presence of the copolymer and of the thermoplastic polymer.
Under such circumstances, shear is created by kneading the copolymer and the thermoplastic polymer during a xe2x80x9ccompoundingxe2x80x9d step of preparing the mixture, e.g. in an extruder having two contra-rotating screws, or in an internal mixer.
The addition of a peroxide or some other cross-linking agent then enables the compatibility of the various polymers to be improved by lengthening the chains of the polymer and of the copolymer, and thus reducing the number of chains. This reduces the number of cross-linking bridges required for cross-linking the material of the invention.
Dynamic cross-linking is generally initiated at a temperature higher than 150xc2x0 C. and preferably lying in the range 170xc2x0 C. to 230xc2x0 C. and under a large amount of shear, i.e. greater than 20 sxe2x88x921 and preferably lying in the range 50 sxe2x88x921 to 250 sxe2x88x921.
The silane can be incorporated during the compounding, and it is preferably incorporated during the step of extruding the composition. Thereafter, cross-linking which has started under the above-specified temperature and shear is then generally allowed to continue in ambient air. Cross-linking is allowed to continue in ambient air for a period lying in the range a few hours to a few days.
Finished Products
The dynamic cross-linking is preferably performed under conditions that make it possible to obtain a cross-linked composition having hardness on the Shore A scale of less than 95.
The composition of the invention is advantageously extruded in a manner that is appropriate for producing a variety of semifinished products which, once cross-linking has been completed, become finished products benefitting from the mechanical properties and the ability to withstand high temperatures that are possessed by the composition of the invention once cross-linked.
Examples of such finished products include power cables in which the insulation and/or the sheath is constituted by the cross-linked composition of the invention.
The insulation and/or the sheath does not present significant elongation after 15 min at 200xc2x0 C. under 0.2 MPa of stress, conditions defined by standard NF EN 60811-2-1, after being stored in air without taking special precautions for one week.
Their hardness on the Shore A scale is preferably less than 95; their breaking elongation is preferably greater than 130%, and more preferably not less than 150%. Their ultimate tensile strength is preferably greater than 7 MPa.